1. Field of the Invention
The present invention relates to a focus control circuit used for a cathode ray tube (CRT) display apparatus such as multi-scan type CRTs.
2. Description of the Related Art
In multi-scan type CRTs, a dynamic focus circuit is required to correct deviation in focus.
A first prior art dynamic focus circuit applies a parabolic wave generating circuit for a pin cushion distortion correction for a color CRT to a horizontal parabolic wave generating circuit and a vertical parabolic wave generating circuit, so that the location of an electron beam corresponds to parabolic waves, thus realizing a multi-scan type CRT (see JP-A-1-132282).
In the first prior art dynamic focus circuit, however, in a multi-scan type CRT, the time period from a start edge of a horizontal synchronization signal (or a vertical synchronization signal) to a start timing of a video signal time period has to be changed in accordance with the scanning frequencies, so that the circuit therefor becomes complex, which increases the manufacturing cost.
In a second dynamic focus circuit, a first triangular wave signal is generated to correspond to a vertical synchronization signal, so that an intermediate level of the first triangular wave signal is caused to be a ground level. Also, a second triangular wave signal is generated to correspond to a horizontal synchronization signal, so that an intermediate level of the second triangular wave signal is caused to be the ground level. Then, a square value of the first triangular wave signal is added to a square value of the second triangular signal, to obtain a focus control signal (see JP-A-4-114589).
In the second prior art dynamic focus circuit, however, a pulse width adjusting circuit is provided to receive the horizontal synchronization signal, so that the time period from a start edge of the horizontal synchronization signal to a start timing of a video signal period is adjusted. Also, a pulse width adjusting circuit is provided to receive the vertical synchronization signal, so that the time from a start edge of the vertical synchronization signal to a start timing of a video signal period is adjusted. Such pulse width adjusting circuits are very complex to respond to the scanning frequencies, which increases the manufacturing cost.
In a third prior art dynamic focus circuit, a time period from the start edge of a horizontal synchronization signal to a start timing of a video signal time period is preset, and a first triangular wave signal is generated on the basis of a delayed timing when this time period passes after a horizontal synchronization signal is generated. Then, a horizontal parabolic wave signal is generated in accordance with the first triangular wave signal using an integration circuit. On the other hand, a time period from the start edge of a vertical synchronization signal to a start timing of a video signal time period is preset, and a second triangular wave signal is generated on the basis of a delayed timing when this time period passes after a vertical synchronization signal is generated. Then, a vertical parabolic wave signal is generated in accordance with the second triangular wave signal using an integration circuit. Then, the horizontal parabolic wave signal is added to the vertical parabolic wave signal, to obtain a focus control signal (see JP-A-63-260365).
Even, in the third prior art dynamic focus circuit, the pulse width adjusting circuits of the second prior art dynamic focus circuit are necessary, which increases the manufacturing cost.
A fourth prior art dynamic focus circuit includes a parameter circuit for storing a plurality of focus characteristic parameters. Thus, one of the focus characteristic parameters is selected to respond to a multi-scan type CRT (see JP-A-63-214791 and JP-A-5-300395).
In the fourth prior art dynamic focus circuit, a memory for storing the focus characteristic parameters becomes large, which increases the manufacturing cost.